The present invention relates to a digital broadcast receiving device mounted on vehicles or other mobile bodies.
Terrestrial digital broadcasting is scheduled to start in the immediate future. Terrestrial digital broadcasting involves hierarchical transmission in which a transmission band allocated to each broadcasting station is divided into a multiple of segments and data is transmitted simultaneously in plural layers, each of which is composed of one or more segments. In hierarchical transmission, more segments constituting a layer make it possible to transmit images of higher resolution but make it more difficult to receive data in vehicles or the like while on the move, without data error. In order to address this problem, for example, ISDB-T, which is standard in Japan, is employed. The ISDB-T divides a 1-channel digital broadcast band into thirteen segments, uses twelve of the segments to provide a high resolution image to a stationary receiver, and uses the remaining one segment to provide a low resolution image through a transmission method that is resistant to transmission error.
This enables stationary receivers installed in homes or other places to receive digital broadcasting of high resolution while enabling mobile receivers such as cellular phones and PDAs on the move to receive low resolution but stable digital broadcasting.
However, users of vehicle-mounted navigation devices and other similar mobile receivers that are mounted on mobile bodies and that have larger screens than those in cellular phones and PDAs wish to see as much as possible, even while on the move, a high resolution image displayed using twelve segments.
Japanese Patent Application Laid-open Publication No. 2003-274303 (hereinafter referred to as Patent Document 1) discloses an example of how to fulfill this wish. A vehicle-mounted digital broadcast receiving device according to Patent Document 1 has a database that is obtained by measuring in advance a digital broadcast signal reception at various points on a map. While a navigation function of the device is showing a route to a destination, the device retrieves from the database the digital broadcast signal reception rate along the route and presents the reception rate to a user, thereby providing the user with information to decide whether to receive a high resolution image with the use of twelve segments or an image that is low resolution but is suitable for reception by moving receivers with the use of one segment.
Patent Document 1, however, merely presents a pre-measured reception rate to the user, and it is the user that switches between reception of a high resolution image with the use of twelve segments and reception of a low resolution image with the use of one segment, thereby burdening the user with a cumbersome operation. Accordingly, the user may have passed a poor reception area before switching to low resolution image reception, or the user may forget to switch to reception of a high resolution image with the use of twelve segments and stick to reception of a low resolution image with the use of one segment after a poor reception area is replaced by a good reception area.
Moreover, when radio waves are received by a mobile body such as a vehicle, the distance from an object that reflects radio waves changes as the mobile body moves and the intensity of radio waves received by the mobile body fluctuates rapidly. This means that, even at a point where the reception is high enough to receive a high resolution image with the use of twelve segments while the mobile body is stationary, there is no guarantee that the high resolution image can be displayed without a break while the mobile body is on the move. Accordingly, a user on the move may give up receiving a high resolution image that can be disturbed or broken, and stick to reception of a low resolution image with the use of one segment.